In standard cosmology, the global dynamics of the universe is assumed to be described by a homogeneous and isotropic FLRW universe model, but a realistic universe model should include local inhomogeneities, and the physical properties of such a realistic model averaged over a sufficiently large scale do not necessarily coincide with those of the FLRW universe. In this article, we give an overview of works studying how local inhomogeneities affect the global expansion. We also point out that, in the framework of averaging inhomogeneities, a natural measure of inhomogeneity is derived, which is known as the relative information entropy in information theory. The temporal behavior of the measure and the relation to the Weyl curvature invariant are explored using the linear perturbation theory of a spatially flat FLRW universe.